My network design is hub and spoke, all offices connected to HQ. Each office has a backup domain controller which replicates with my PDC. All DCs are running Windows Server 2003. I plan on upgrading my PDC to Windows 2003 R2 and then to Windows Server 2008. Do I need to upgrade all the other DCs at the same time? If I upgrade to WS2K R2 only, can I upgrade the others at a later date?
You can join an Windows Server 2008 domain controller onto your Windows 2003 Active Directory domain as it sits. You won't be able to change the "Functional Level" of the domain or forest to Windows 2008 until you've upgraded all the Windows 2003 DC's.
You should be fine upgrading the DC. Just be sure to not Upgrade the operational mode of your AD until all servers are at the same level.
I added a Windows 2008 Server as a secondary domain controller about six months ago without problem. My primary domain controller remains a Windows 2003 Server. I did, however, upgrade the operation mode of our active directory as Zypher mentioned.